The present invention generally relates to an apparatus for fail-safely disconnecting an electrical load, and more particularly to an apparatus having a first and at least one second safety switching device. The invention also relates to safety switching devices for use in such an apparatus.
Safety switching devices are particularly used in industrial production environments in order to reliably and safely disconnect machines, such as a hydraulic press or a milling machine, when this is necessary for the safety of operators, or for other reasons. Safety switching devices of this generic type typically have one or more input terminals, to which input devices such as emergency-stop buttons, two-hand operator buttons, guard doors or light barriers can be connected. An evaluation and control unit evaluates the input signals from these input devices in a failsafe manner and produces a switching signal on an output side as a function of them, once again in a failsafe manner. The switching signal controls actuators, in particular contactors, by means of which the machine is disconnected, once again in a failsafe manner.
The term “failsafe” in this context means that disconnection of the machine must be ensured in all circumstances, and even when a functional fault occurs within the chain comprising the input device, the safety switching device and the actuator. Safety switching devices must therefore be designed such that they ensure that the machine being monitored is always in a safe state, even in the event of failure of a component, in the event of cable breaks, or in the event of other malfunctions. Accordingly, the term safety switching device in this context refers only to those devices and appliances which comply at least with Category 3 of European Standard EN 954-1, or comparable requirements.
Conventionally, safety switching devices of this generic type have until now and are still often designed with discrete components and with electromechanical switching elements (relays). Only in the last few years have changes been made to designing safety switching devices which also make use of electronic components and, in particular, electronic switching elements (bipolar and/or field-effect transistors). WO 01/67610 A1 discloses safety switching devices and apparatuses with combined safety switching devices which produce potential-related switching signals at the output. In contrast to this, conventional safety switching devices use floating outputs. Owing to the potential-related outputs, it is no longer useful to connect the outputs of two or more safety switching devices according to WO 01/67610 A1 in series with one another in order to implement an apparatus comprising a plurality of safety switching devices. WO 01/67610 A1 therefore proposes a new approach of combining safety switching devices having potential-related outputs. In this approach, the potential-related outputs of a first safety switching device are connected to specific inputs of a downstream, second safety switching device. This results in an apparatus which can be used in a very flexible and versatile manner. However, in the case of the apparatus of WO 01/67610 A1, at least two potential-related outputs of the first safety switching device have to be connected in parallel with at least two corresponding inputs of the second safety switching device in order to achieve maximum failsafety.
DE 100 37 737 A1 discloses a method and an apparatus for safe single-channel evaluation of sensor signals. In this case, specific redundant signals are derived from redundantly produced sensor signals. All the signals are then digitized, are converted to a single-channel digital signal, and are transmitted to a higher-level processing unit. However, this method is not suitable for a combination of two or more safety switching devices of the type described above to form a system of safety switching devices as is known from WO 01/67610 A1.